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In this literature review is discussed what is already known about the effect of flavour in creep feed and the whole mechanism around flavour (smell, taste, saliva production, enzymes, digestion, etc.). Information in literature is collected, compared, judged and evaluated and summarized in this review. Different literature resources are used, such as scientific articles and books that provide useful information about the topic.
First are the pathways of taste and smell to the brain described, how this works and what factors have an influence. After that the conclusions of several studies that used flavour in the creep feed of piglets are summarized. Then the different steps in the relation between flavour of creep feed and diarrhoea is tried to describe (see figure 1). At the end new research questions are formulated based on the missing information that could not be found in the literature. In the next phase a research proposal will be developed for an experiment to answer the questions.
Peripheral sensing in pigs
The five senses; vision, hearing, smell, taste and somatosensing have an effect on the preference and feed intake by pigs (Tedó, 2009). The perception of food, affected by physiologic and external contextual factors, largely determine palatability (Hyde and Witherly, 1993). The ability to sense the nutritional value of available food sources stimulates appetite for appropriate ingredients which results in a successful self-nourishment mechanisms (Hyde and Witherly,1993; Shepherd, 2006; Roura and Tedó, 2009; Tedó, 2009). The entrance of food in the oral cavity and the olfactory perception of food, orthonasal (external air) and retronasal (air expired from the oral cavity), evokes the peripheral senses known as somatosensing (based on thermal or mechanical sensations), smell (detection of odorants and pheromones) and taste (Shepherd, 2006; Roura and Tedó, 2009; Tedó, 2009). So, sense of food stimulate the oral and nasal cavities which play a role in signalling food arrival to the rest of the digestive system and as a consequence digestive secretions and gut motility are stimulated (Roura and Tedó, 2009).
Table 1. The twelve cranial nerves of the peripheral nervous system and functions (Roura and Tedó, 2009).
Evoking of the peripheral senses triggers the peripheral nervous system. The peripheral nervous system consists of twelve cranial nerves (table 1), which are visible on the ventral surface, are directly connected with the brain (figure 1).
Figure1. Cranial nerves visible on the ventral surface of the brain (bron boek).
The cranial nerves of the peripheral nervous system used for determining somatosensing, smell and taste are olfactory (cranial nerve I for the sense of smell), Trigeminal (cranial nerve V for somatosensing), Facial (cranial nerve VII for taste), Glossophayngeal (cranial nerve IX for taste) and the Vagus (cranial nerve for taste) (figure 2).
Figure 2. Cranial nerves of the peripheral nervous system in the pig (drawings by Joaquim Roura, 2009).
Now the mechanism of smell and taste will be described. Although these two are related to each other, they will be described separately to give a better view of how they work.
The sense of smell, or the olfactory system in vertebrates is able to discriminate a wide array of structurally diverse odorants (Buck, 1996). The ability to detect diverse odorants depends on the direction of the air flow through the nasal cavity. The direction of the air flow can be divided into two categories (figure 3): orthanasal (air perceived through the nose) and retronasal (air perceived through the mouth) (Shepherd, 2006; Roura and Tedó, 2009; Tedó, 2009). Assumed is that the air flow in humans and the pathway to the brain is the same for pigs, because of humans this is described by Shepherd (2006) and of pigs by Roura et Telo (2009) and there are relatively similar.
Figure 3. Perception of the sense of smell. a, orthanasal olfaction (and the brain systems which is involved in smell perception) b, retronasal olfaction ( and the brain systems which is involved in smell perception) (Shepherd, 2006).
Research of Roura and Tedó (2009) shows that the oral and nasal cavity is connected to the central nervous system (brain and spinal cord) through the peripheral nervous system. The cranial nerve which transport the sense of smell to the brain, is cranial nerve I (the Olfactory nerve) (table 1, figure 2).
The detection of chemically distinct odorants presumably results from the association of odorous ligands with specific receptors on olfactory neurons, which stay in a specialized epithelium in the nose (Buck and Axel, 1991; Buck, 1996; Schipley and Ennis, 1996). The olfactory epithelium is located in the upper wall of the nasal cavity. The olfactory epithelium consist of three cell types (figure 4); the olfactory sensory neurons (5), the supporting cells(4) and a basal layer of stem-cells (6). Also the olfactory epithelium consists of turbinates which provide an increased epithelial surface area (Buck and Axel, 1990; Buck, 1996; Reed, 1990; Roura and Tedó, 2009; Tedó, 2009).
The olfactory sensory neurons are bipolar neurons with a single dendrite that reaches the lumen (1) of the nasal cavity through the mucosa. There it forms a swelling, the olfactory knob bearing 20 to 30 olfactory cilia (2) that lie in the surface of the olfactory epithelium (12) allowing it to come in contact with odorants dissolved in the thin layer of mucus (3) (Roura and Tedó, 2009; Tedó, 2009; Moulton and Beidler, 1967). On the plasma membrane of olfactory cilia lie olfactory receptors which are responsible for the recognition of odorous ligands. Olfactory receptors can be divided into two different classes: one class related to fish-like receptors that bind water-soluble odorants and one
class where the receptors bind to air-born molecules (Roura and Tedó, 2009; Tedó, 2009). When odorous ligands bind to olfactory receptors the chemical stimulus is convert into an electrical stimulus and transported through the olfactory bulb to the central nervous system. Research of Buck and Axel (1991) and Roura and Tedó (2009) showed that the olfactory receptors on the olfactory cilia belong to a superfamiliy of receptors known as GPCR (Guanine-nucleotide-binding (G) protein-coupled receptors).
Figure 4. The Olfactory epithelium and Olfactory bulb
(drawing by Joaquim Roura, 2009)
When an odorants ligand enters orthanasal or retronasal the nasal cavity it binds to an odor-specific trans membrane receptor (figure 5). This binding results in the interaction of the trans membrane receptor with a GTP-binding protein (Gs(olf)) which in turn leads to the release of the GTP-coupled α subunit of the G protein. Release of the GTP-coupled α subunit stimulates adenylyl cyclase to produce elevated levels of cAMP. The increase on cAMP opens cyclic nucleotide-gated cation channels and causing an alteration in membrane potential (Buck and Axel, 1991; Buck, 1996; Reed, 1990).
In general the G protein initiates a cascade of intracellular signalling events which leads to the generation of an action potential that is propagated along the olfactory neuron axon to the olfactory bulb (Buck and Axel, 1991; Roura and Tedó, 2009).
Figure 5 . A pathway of olfactory signal transduction (Buck and Axel, 1991).
The olfactory bulb contains glomeruli (9) and mitral cells (6). Each odorant is identified by a specific activity pattern in the glomerular layer of the olfactory bulb. From there axons of the mitral cells connect the glomeruli to the olfactory cortex where information is organized and sent to other areas of the brain (Roura and Tedó, 2009; Tedó, 2009; geneeskunde boek).
If a piglet takes feed in his mouth he will taste it. Based on the taste a piglet can react by eating more or stop eating. In this part there will be explained how this reaction occurs on a feed and why.
First the definition of taste, 'taste is a group of sensations in the oral cavity that allows animals to identify nutrients and anti-nutritional compounds'(Bachmanov and Beauchamp, 2007; Dulac, 2000;). There are five primary taste activities, sweet, bitter, sour, salty and umami. Sweet taste identifies carbohydrates, umami identifies amino acids, salt taste targets proper dietary electrolyte balance and sour and bitter warn against the intake of potentially noxious and or poisonous chemicals( Chandrashekar et al.)
Chemosensory cells called taste buds detect stimuli from the food. The mouth papillae contain several thousand of taste buds. Taste buds are primarily found on the tongue grouped in three kinds of papillae, fungiform, foliate and circumvallate. Fungiform papillae are found on the tip of the tong and innervated by the facial branch of the cranial nerve VII(chorda tympani, CT). Circumvallate and foliate papillae are on the back of the tong and are innervated by the cranial nerve IX(glossopharyngeal, GP). ( Danilova et al., 1999;Kumar and Bate, 2004)
Taste buds consist of taste receptor cells( TRCs), wich are modified epithelial cells. Located on the membranes of the microvilli of these cells are the taste receptors that recognize specific soluble taste ligands( Gilbertson et al., 2000)
Main taste categories Salt, sour, bitter, sweet and umami.
Each TRC type has its own distinct pathway by which the taste receptors are activated. Salt and sour compounds activate TRCs through ion channels in the apical cell membrane. Bitter, sweet and umami compounds act through more specific trans membrane taste receptors(Sugita, 2006; Gilbertson 2000)
Salt taste is mainly stimulated by sodium, which maintains the ion and water homeostasis. There are two types of receptors for salt taste: amiloride- sensitive and amiloride- insensitive. Hellekant and Danilova( 1999) showed that pigs give no different reaction to NaCl if amiloride is added, this suggest that pigs only have amiloride-insensitive receptors. Amiloride- insensitive, are associated to other fibers from the CT and most of the GP taste nerve responses. These are mainly expressed in foliate and circumvallate taste cell papillae. These channels are derived from the vanilloid receptor-1(VR-1) that is responsible for response to K+, NH4+ and Ca+ salts( Lyall et al., 2004). Through the channel there is a cation influx which elicits membrane depolarization leading to action potentials( Fig 1)( Sugita 2006).
Sour taste responses are proportional to proton concentration. It servers to detect unripe and spoiled food and to avoid tissue damage by acids and problems of systemic acid-base regulation. There are different kinds of receptors for sour taste but all work through proton-gated channels, proton-conducting channels of PH-dependent ion exchangers in the cell membrane. When a channel is activated depolarization follows through the ion channels or by modulation of the intracellular concentration of ions and creation of an action potential may follow. The depolarization may be associated with production of an action potential through activation of voltage-gated channels, resulting in the release of neurotransmitter onto an afferent nerve fiber( Fig. 1)(Lindemann, 2001; Sugita, 2006).
Fig 1. Reaction pathway for salt and sour taste(Sugita, 2006).
Bitter, sweet and umami taste receptors belong to a superfamily named after guanine-coupled-nucleotide-binding protein coupled receptors(GPCRs). Taste GPCRs are divided into two families: T1R and T2R(; Dulac, 2000; Bachmanov and Beachamp, 2007; Sugita 2006).
The T1Rs receptor family possess a large N terminal extracellular domain and generate at least two heteromeric receptors: the T1R1/T1R3 for umami and the T1R2/T1R3 for sweet tastes( hoon et al., 1999; Nelson et al., 2001). The T2R family is defined as the bitter taste receptor.( Adler et al., 2000)
Umami taste is perceived by the T1R1/T1R3 and other metabotropic glutamate receptors, mGlu1 And mGLu4( nelson et al., 2002). The umami taste is mainly related to protein, peptides and L-amino acids (Ninomiya, 2002; Bachmanov and Beachamp, 2007)
Heterodimer T1R2/T1R3 is the only known sweetness receptor( Margolskee, 2002). This sweet taste receptor heterodimer recognizes a large collection of diverse chemical structures like sugars, some D- amino acids, artificial sweeteners and some proteins, this can differ between animals(Nelson et al. 2002; Bachmanov and Beachamp, 2007).
Bitter, sweet and umami taste have a common transduction pathway that starts with a ligand binding to the taste receptors. Then through Gα to activate PLCβ2-dependent pathway, which catalyzes the formation of inositol triphosphate(IP3) and Dyacilglicerol(DAG). These lead to the release of calcium from intracellular storages( Fig. 1). The pathway for sucrose and other sugars is a little different, binding activates Gαs which activates adenylys cyclase to generate cAMP. This can cause direct a cation influx to cNMP-gated channels or indirectly to activate protein kinase A, which phosphorylates a K+ channel, leading to closure of the channel and depolarization of the taste cell. The depolarization leads to a voltage-dependent Ca2+ influx(margolskee, 2002; sugita 2006). The intracellular calcium release activates the TRPM5 channels and results in the entry of Na+ and membrane depolarization which might be required for generation of action potential of taste(Fig.1)(Sugita, 2006).
Fig. 2. Reaction pathway for bitter, sweet and umami tastants( Sugita, 2006).
There is a genetic variation and common polymorphisms of T1R's within and between population These may explain differences in dietary preferences and food selection. (Reed et al., 2006; Garcia-balio et al., 2008)
Specific tastes in pigs
Hellekant and danilova preformed research on different tastes in pigs. The activity of the CT and the GP with different stimuli. Also from which clusters the activity contains it can be deducted if the reaction is positive or negative. Acids gave high activity, this indicates that acids give a distinct taste to the pig. It could not be deducted if the reaction is positive or negative. Alitame, aspartame, cyclamate, super-aspartame, thaumatin gave no response so it lacks sweetness to pigs. Sucrose, glucose and, to a lesser extent, lactose proved to give a positive reaction. Glycine elicit strong responses in the positive fibers, thus it seems that is attractive for the pig. On the other hand, it also was active in the negative fibers which will deter its attractiveness. Umami compounds turn out to be a powerful tastant to pigs. Also the analysis positioned it with sweeteners which give a positive reaction. From NaCl stimuli it was found that the pig has a low ability to taste NaCl(Hellekant and Danilova, 1999). Also Tedó( 2009) found preference of pigs for potential umami tastants. Nelson and sanregret (1997) found that pigs do react aversively to compounds that humans find bitter-tasting, although at different concentrations then humans do.
Concluding taste can have an important influence on food selection and intake. Pigs taste some compounds different than humans do, so feed additives for flavour have to be tested on pigs to know the reaction and preference. From research is shown that pigs prefer umami and sweet tastants and do not like bitter tastants.
Studies about effects of flavour
To attract the interest of young pigs in solid food flavouring ingredients like sugar are used. Although the piglets prefer flavoured food when they have the choice with unflavoured food, this doesn't mean that the feed intake will be increased when there is only the preferred food available (Forbes, 1998).
King (1979) did a study where 50 ppm of Firanor no. 24 (a natural, synthetic and artificial chemical composition used for flavouring animal feeds) was added to the diet of the piglets. The conclusion was that the addition of flavour to creep feed does not significantly improve the creep feed intake (flavoured vs. control: 51.8 vs. 43.9 g/d) or growth rate (208 vs. 206 g/d). Millet et al. (2008) concluded the same, after a study with a flavour containing sodium saccharinate, glucose and concentrated flavour based on cinnamon, anise and caramel extracts (at a concentration of 1.5 g/kg). Their results for day 4-8 after weaning (relatively flavoured vs. control) were a feed intake of 287 vs. 312 g/d and a weight gain of 132 vs. 126 g/d. Kornegay et al. (1979) also showed no improvement of daily feed intake (262 g for flavoured vs. 267 g for control), feed conversion (169 vs. 166 g) or daily gain (155 vs. 161 g) after addition of various feed flavours (sugar replacers and aromatic attractants).
Another study with the addition of the feed flavour Luctarom (a natural sweetener, equivalent in sweetness to saccharin) showed that this did not affect preweaning performance (total body weight gain day 18-21 flavoured vs. control: 8.8 vs. 8.9 kg) or the proportion of piglets eating creep feed (Sulabo et al., 2010).
McLaughlin et al. (1983) did an experiment where piglets could choose from flavoured and unflavoured food in a T-maze. Then two flavours (sweet caramel flavour and cheesy flavour) were selected to test the effect on performance after weaning. The groups with flavoured feed had an increased feed intake and body weight gain during the first week compared to the control group (relatively 136 vs. 103 g/d and 75 vs. 29 g/d).
Kennedy and Baldwin (1972) showed that pigs have a preference for solutions of sucrose, glucose and saccharin, compared to water.
The adding of bitrex (a very bitter substance) to the food of piglets caused an abrupt decrease in feed intake. However, when the piglets were forced to eat the food because of hunger, they discovered that the food was not metabolically harmful and began to eat normal amounts (Forbes, 1998).
Another study by Hines (1973) showed that piglets consume more flavoured diet when they are given a choice between flavoured and unflavoured diets. This preference for flavoured creep feed continued after weaning, piglets consumed 1.8 times more of the flavoured diet than of the control diet. The average daily feed intake and weight gain did not differ between post-weaned pigs with either flavoured or control diet (relatively 2.53 vs. 2.50 lbs and 1.24 vs. 1.21 lbs). The flavour used in this study is Pig Krave, supplied by Feed Flavors, Inc., Wheeling, Illinios, but it is not known what kind of flavour this is.
A different kind of experiment is done by Langendijk et al. (2007). In this study the sows also received a flavoured diet to investigate the effect of prenatal exposure of flavour and litters were submitted to intermittent suckling. During lactation the litters received creep feed with 40 gram garlic and 20 gram aniseed added. The feed intake after weaning was higher for litters weaned at six weeks that received flavoured (833 vs. 687 g/d). Adding flavour to the diets did not affect the post-weaning weight gain, although post-weaning adaptation and acceptance of creep feed is improved by early experience with flavours.
Brain to maagdarmstelsel/ mouth to maagdarmstelsel
Saliva is a mix of secretions from several glands in the oral cavity, saliva contains electrolytes, mucins, antibacterial compounds, enzymes and other functional proteins. Von Ebner's glands (VEG) produce enzymes with a lipase activity and carrier proteins. Another function of the VEG is to clean and coat the oral cavity and to facilitate swallowing (Roura and Tedó, 2009).The exact composition of the saliva differs per species. Chauncey et al (1963) showed the presence of acid phosphatase, non-specific esterases, pseudo cholinesterase, β-d-galactosidase and amylase in pig saliva. Hudman et al. (1957) examined the amylase and maltase activities in piglets, from 1 to 42 days of age. They concluded that the maltase activity is of little significance and that the saliva of a piglet is comparatively low in amylase activity.
Saliva leads to the digestion of starch and dissolving of food particles, and the production of saliva is essential for taste. The two main functions of taste are the promotion or inhibition of ingestion and the preparation of the body to respond to the ingested materials. Cephalic phase responses can inhibit feed or prepare an animal to deal with toxic food. Gastric motility can for example be decreased by a bitter tasting substance. Some cephalic responses are specific to the nutritional properties of a tastant, for example responses to sweet substances are different from the responses to bitter substances. Cephalic phase responses play a role in appetite and satiety and are increased by adding smell and taste to the food. Food that is considered as tasteful leads to more robust cephalic phase responses and therefore to larger meals (Power and Schulkin, 2008). So the adding of taste to creep feed that piglets find attractive could lead to a higher feed intake.
Saliva is important for the ingestion and digestion of food. Table 1 shows multiple functions of saliva. Saliva production is caused by taste, smell and sight. On the other hand, saliva is essential for taste perception. A lack of saliva could lead to less taste perception, problems with swallowing, less digestion of starch and lipid and more microbial activity. The saliva stimulation is highest with sour taste, followed by salt, sweet and bitter taste. The degree of adaptation caused by continuous taste stimulation is highest for sweet taste and lowest for sour taste (Pedersen et al., 2002).
Table 1. Multiple functions of saliva exerted in the upper part of the gastrointestinal tract and especially in the mouth (Pedersen et al., 2002)
A brush border exists of many microvilli in the small intestine, because of these microvilli the surface of the small intestine increases and so the absorption of nutrients. On the brush border many enzymes exist, like lactase, maltase, sucrose…. These enzymes degrade nutrients (Wordt vervolgd)
Maagdarmstelsel influence diarrhoea
As mentioned earlier, piglets have a preference for sweet flavours like sugar. Sugar beet pulp is very sweet and therefore probably attractive for piglets. Sugar beet pulp is also high in fermentable Non-Starch Polysaccharides (NSP). Newly weaned pigs appear to have considerable capacity to digest NSP, especially from sugar beet pulp (Pluske et al., 2000). So the feeding of sugar beet pulp could have a good influence on the attractiveness as well as the digestion of the food by young pigs. Lizardo et al. (1997) also showed that sugar beet pulp in diets has a positive effect on the development of the digestive function and on the performance of the weaning piglet.
Questions derived from the literature study
What is the relation between saliva production and digestion?
Does saliva have a positive effect on the digestion in pigs?